Waveguides Bends and Twists

The size, shape, and dielectric material of a waveguide must be constant throughout
its length for energy to move from one end to the other without reflections. Any abrupt
change in its size or shape can cause reflections and a loss in overall efficiency.
When such a change is necessary, the bends, twists, and joints of the waveguides must
meet certain conditions to prevent reflections.

Bends

Waveguides may be bent in several ways that do not cause reflections. One way is
the gradual bend shown in the right part of the following figure. This gradual bend
is known as an E bend because it distorts the E fields. The E bend must have a
radius greater than two wavelengths to prevent reflections.

H - bend

E - bend

Figure 1: Waveguide bends

Another common bend is the gradual H bend shown in the leftt part of the figure.
It is called an H bend because the H fields are distorted when a waveguide is bent
in this manner. Again, the radius of the bend must be greater than two wavelengths
to prevent reflections.

A sharp bend in either dimension may be used if it meets certain requirements.
Notice the two 45-degree bends in figure; the bends are 1/4·λ apart. The
reflections that occur at the 45-degree bends cancel each other, leaving the fields
as though no reflections have occurred.

Figure 2: Waveguide bends

Twists

Sometimes the electromagnetic fields must be rotated so that they are in the proper phase
to match the phase of the load. This may be accomplished by twisting the waveguide as shown
in the figure. The twist must be gradual and greater than two wavelengths (2·λ).

Figure 3: Waveguide twist

Figure 3: Waveguide twist

The flexible waveguide allows special bends which some equipment applications might
require. It consists of a specially wound ribbon of conductive material, most commonly
brass, with the inner surface plated with chromium. Power losses are greater in the
flexible waveguide because the inner surfaces are not perfectly smooth. Therefore,
it is only used in short sections where no other reasonable solution is available.